The present invention relates to the field of controlling crystal growth. More particularly, the present invention relates to a device for controlling crystal growth processes in which different process phases of the overall process can be distinguished, for instance, the phases of the melting process for the starting material or of the cooling of the residual melt or the phases during which the neck, the shoulder or the body of the crystal are subjected to a defined control strategy. Measured values such as the diameter of the crystal neck are detected during these phases and are fed to an evaluation device which influences the process variables such as the heating temperature of a crucible. In a detailed aspect of the invention, the evaluation device comprises an allocation device which allocates each process variable measured during a phase, as the neck diameter of a crystal, for instance, with a defined process variable. Such process variables include for instance, temperature of the crucible or crystal turning velocity.
A monocrystal is understood to mean a single homogeneous crystal, the atoms of which are arranged homogeneously in a three-dimensional lattice. Monocrystals thus differ from other materials, such as polycrystalline or amorphous bodies, by their regular structure, which is also evident from external observation.
In order to grow such monocrystals, the appropriate materials such as germanium or silicon are first melted and then crystals are obtained from this melt by certain processes.
In most of these processes it is a prerequisite that either only one seed forms or one seed grows preferably rapidly. Rod-shaped monocrystals are produced as a rule by solidification of the melt in a temperature gradient. In these processes, including the so-called 30Bridgman method, the melt is located in a crucible which is moved at a slow speed adapted to the crystallization rate of the respective material through a temperature gradient of 10-100.degree./cm including the melting point.
So-called zone melting is also applied as a modification of this principle.
For drawing out of the melt, according to the so-called "Czochralski method", a small monocrystalline piece of the material, the seed crystal, is dipped into the melt and, after adjustment of the temperature equilibrium at the boundary surface between liquid and solid, is drawn out at a uniform rate. This drawing rate is controlled. In the process, additional material constantly crystallizes onto the lower end of the incipient monocrystal.
The physical parameters with which the growth of such a monocrystal can be influenced are, for instance, the temperature of the melt or the drawing rate.
In order for the aforementioned or other monocrystal-producing processes to run properly, special regulation methods or devices are necessary.
For example, in order to regulate the cross section of the grown crystal in a Czochralski process, a device is already known with which a first signal, corresponding to the inertial mass of the crystal, is compared to a second signal that corresponds to a reference signal (British Patent No. 1,457,275). This reference signal corresponds at any time to the expected value for the first signal. The deviation value between the two signals is called on for the regulation of the cross section. The heating for the melt and an electric motor for driving a crystal-raising rod are also controlled.
It is disadvantageous in this regulation method that only one parameter can be controlled with it, namely, the cross section of the drawn crystal. Regulation of the different areas in a crystal, for instance, the neck or the shoulder, is not possible with the known method.
Also known is a method for controlling the growth of a crystal in which the growth is determined by a set of measurable and nonmeasurable variables (published European Patent Application No. 0,821,082). This method includes preparing an on-line simulation software with a reduced number of variables, the reduction in the number of variables being obtained by the utilization of a projection algorithm. This software is then accelerated in that a database is generated, in which values of variables calculated off-line are stored. Thereafter the on-line software is adapted to the results which are obtained by the off-line simulation and by measurements by adjusting the results which were predicted by the on-line simulations. Then a control loop is formed and at least one of the variables is regulated in real time, where the control loop uses the accelerated and adapted on-line simulation as an online observer. The measurable and nonmeasurable variables here include drawing rate, heating power, temperature distribution in the crystal, melt flow characteristics, temperature distribution on the inner surface of the quartz crucible, temperature distribution on the surface of the melt, the shape of the solid-liquid boundary and the vaporization of SiO. Disadvantageous in this method is the fact that different strategies are applied in the regulation of different sections of a crystal, for instance, the neck or the body.
Therefore, it is an object of the present invention to have the same regulation strategies for all different areas of a crystal.